This invention relates to transducers for the transmission of sonic energy and more particularly to a self-biased transducer having drive stacks comprised of interleaved high remanent flux magnets and high-strain magnetostrictive material for producing sonic energy of high power and with high performance.
Magnetostrictive lanthanide alloys, such as Terfenol-D (i.e. Tb.sub.0.3 Dy.sub.0.7 Fe.sub.1.9), are capable of producing over five times the rms strain developed by the most competitive piezoceramics and ten times the rms strain developed by the most competitive non-lanthanide (i.e. nickel) magnetostrictive alloys. Since the acoustic output power generated by an underwater transducer is proportional to the square of the strain, this can result in a large advantage in power producing capability. Additionally, lanthanides such as Terfenol-D have lower sound speeds than non-lanthanide magnetostrictive alloys and piezoceramics and posess seventeen times the thermal conductivity of piezoceramics such as lead-ziconate-titanate. The low sound speed tends to improve bandwidth and lowers resonance frequency, while the high thermal conductivity tends to improve power handling ability and increases attainable duty cycle. The degree to which these advantages can be adequately exploited depends upon overcoming several problems which arise in transducer design because of certain intrinsic properties of the lanthanide material.
Terfenol-D has a relative permeability of only four or five, at least an order of magnitude lower than magnetostrictive nickel alloys. In addition, for purposes of economy of length and cost effectiveness, relatively short lengths of the material are used. The combination of low permeability and low length-to-diameter ratio of the Terfenol-D rods resulted in non-uniform bias and drive fields in prior-art transducer designs which led to utilization of only a portion of the material and non-uniform strain in the material. Demagnetization effects and fringing flux tend to increase leakage inductance and degrade transducer coupling. The low length-to-diameter ratio also leads to stray flux finding its way into metallic transducer components where eddy currents and hysteresis losses lower conversion efficiency.
Unlike piezoceramics, which are prepoled, Terfenol-D and the other lanthanide alloys require a polarizing field because they are magnetically soft and retain insufficient remanent fields for linear operation. In the case of Terfenol-D, the polarizing field may be supplied by either a coil carrying direct current or permanent magnets.
If DC current is used to supply the bias field, it may be superimposed on the AC drive coil or may be carried on a separate coil. If separate AC and DC power supplies are employed and the currents are superimposed in one coil, a large choke is required to pass DC and avoid driving the DC supply with the AC power source. Also, a large condenser is required to pass AC and block DC current from entering the AC supply. If separate windings carry AC and DC, the requirement for the condenser is removed, but a large burdensome choke in series with the DC current winding is still required since the two coils will be coupled due to the transformer action of two concentric coils magnetically linked by a common core.
The requirement for separate DC and AC power supplies can be avoided by various solid state amplifier approaches, but these approaches similarly suffer from the requirement for bulky magnetics such as heavy autotransformers. Additionally, all DC-current biasing approaches result in a heavy thermal burden being placed on the transducer, eroding much of the potential advantage gained by the drive material's high thermal conductivity. Overall efficiency is also decreased not only by DC power being dissipated in the transducer, but also because of DC transmission losses occurring in the DC power supply cable, especially when the cable runs are long.
The alternative to direct current biasing is biasing with permanent magnets. In the prior art, the cost of permanent magnet biasing is usually decreased AC efficiency, decreased transducer coupling, and overall reduced transducer performance. The AC efficiency is normally reduced because the permanent magnets also have very low permeabilities and therefore increase the reluctance of the magnetic circuit. Hence, greater magnetomotive force (greater AC drive current) is required to achieve a given flux density, resulting in increased coil losses. Since eddy currents in the magnets cause additional losses, the permanent magnet material may have to be shielded with sheet metal to avoid demagnetizing the permanent magnets with the AC drive field. As a result of eddy currents, large amounts of energy ar dissipated in the magnet shielding, which usually results in a catastrophic decrease in efficiency. The low reluctance of the AC magnetic circuit results in greater fringing, which leads to additional losses due to stray flux entering metal transducer parts. Also, large amounts of energy are stored in permanent magnets, thereby increasing leakage inductance, raising the electrical quality factor, and reducing transducer coupling. If the permanent magnets are not placed in an optimum way, the magnetostrictive drive material is likely to be biased non-uniformly, resulting in poor utilization of the drive material with some regions insufficiently biased and others near saturation. Hence, the use of permanent magnets as in the prior art does not in itself guarantee high efficiency or improvement in overall performance.